1. Field of the Invention
The present invention relates to an image display apparatus having a display element in which illumination light is modulated into light showing an image. Particularly, the invention relates to an illumination optical system of the image display apparatus.
2. Description of the Related Art
In recent years, a head mounting type image display apparatus which is mounted to a head and is used in front of eyes spreads as an image display apparatus for personal use. In general, the head mounting type image display apparatus has a display element for displaying an image, and an eyepiece optical system for guiding light of the image displayed on the display element to eyes of a viewer and providing an enlarged virtual image. Although CRT (Cathode-Ray Tube), for example, in which the element itself emits light showing an image is occasionally used as the display element, a liquid crystal display, in which the display element for modulating given illumination light into light showing an image (hereinafter, referred to as an image light) is combined with a small light source (for example, a light emitting diode) for supplying the illumination light, is mostly used in order to miniaturize and lighten the apparatus.
A structure, such that the light source is combined with the display element for modulating the illumination light from the light source, is provided with a diffracting reflection element for diffracting and reflecting the illumination light from the light source so as to guide it to the display element in order that while a degree of freedom of arrangement positions of the light source, the display element and the eyepiece optical system is being heightened, an incident angle of the illumination light with respect to the display element is made to be suitable.
The structure of such an image display apparatus is schematically shown in FIG. 17. The image display apparatus is composed of a liquid crystal display 91 which is the display element, a light emitting diode 92 which is the light source, a diffracting reflection element 93, an eyepiece optical system 94, and a convex lens 95. Divergence light emitted from the light emitting diode 92 as illumination light is changed into approximately parallel light by the convex lens 95. The diffracting reflection element 93 diffracts and reflects the illumination light which is changed into the approximately parallel light by the convex lens 95 at a reflection angle different from the incident angle so as to guide the light to the liquid crystal display 91. The liquid crystal display 91 guides the illumination light in the form of the approximately parallel light as an image light to the eyepiece optical system 94, and the eyepiece optical system 94 provides an enlarged virtual image of the image to an eye E of the viewer. As the diffracting reflection element 93, a diffraction grating or a hologram is used.
A prior image display apparatus for guiding the illumination light to the eyepiece optical system through the diffracting reflection element, however, has a small observation pupil because the illumination light guided to the display element is parallel light or is extremely close to the parallel light. The eye of the viewer is, therefore, easily deviated from the observation pupil of the apparatus due to a change in a relative position between the apparatus and the eye of the viewer, and a part of an image to be observed is omitted or the image cannot be observed completely. The head mounting type image display apparatus desirably provides easy mounting and has agreeability, and it is mostly of a glass type. In this form, however, the apparatus easily moves, thereby a situation where the image cannot be observed arises frequently.
Even if a member for fitting is devised so as not to move the apparatus, since a gap between left and right eyes differs depending on individuals and the relative position between the apparatus and the eye differs depending on viewers, it is difficult to provide the image display apparatus with high general-purpose properties. In order to provide the apparatus with which many people easily observe an image, it is necessary that the entire optical system including the light source through the eyepiece optical system is movable with respect to a mounting member, and thus the structure becomes complicated, and the miniaturization and lightening become difficult.